Opaque readers of the type under consideration in the present invention generally comprise a housing structure with a rear projection screen. Within the housing there are provided powerful lights which will illuminate an opaque document, the reflection from the surface of the document then being magnified and focused by an appropriate projection lens onto the rear of the screen. Generally an intercepting mirror is provided between the projection lens and rear screen surface to direct the light so that the object to be viewed can be received in a flat or horizontal orientation and the viewed image on the screen appear in a vertical plane.
Similar housing arrangements are provided for microfilm or microfiche readers except that the light source is disposed behind the film or fiche to project the image in question.
A desirable feature in both types of readers would be to enable a change in the magnification of the projected image in order that certain details of the document could be studied. In the interest of economy and compactness, such change in magnification can be effected without having to change the basic projection lens. Rather, a simple diopter may be disposed between the lens and the screen to provide the magnified image. However, when the diopter is utilized with the projection lens it is necessary to axially shift the lens in order to maintain the magnified image in focus.
In converting back from a magnified image to a normally projected image, the diopter lens would be removed from in front of the projection lens and the projection lens then shifted back to its initial position to again maintain a focused image.
A desirable feature in any type of opaque or film reader incorporating means for changing the magnification would be a simple manual control for requiring only a single movement for both shifting axially the position of the projection lens and removing or reinserting the diopter lens. Moreover, in any such control device a requirement would be for a fine focusing adjustment once one or the other of the two available magnifications is selected.